Removal of carbon dioxide and/or hydrogen sulfide from gases containing olefines and acetylenes

ABSTRACT

A process for the removal of CO2 and/or H2S from gases containing olefins and acetylenes by absorption in an aqueous solution of an alkali metal salt of amino acid and desorption by decompressing and heating the solvent, which comprises treating the solvent, before or after desorption, with a hydrocarbon mixture substantially consisting of benzene for the purpose of removing impurities and, after said treatment, separating the hydrocarbon mixture from the aqueous solution.

C United States Patent [151 3, John et al. 1 May 2, 1972 54 REMOVAL OFCARBON DIOXIDE 58 Field ofSearch ..23/2, 2.3, 3, 3.3, 4

AND/ OR HYDROGEN SULFIDE FROM GASES CONTAINING OLEFINES AND [561References Cited ACETYLENES UNITED STATES PATENTS Inventors Harald John;Rolf Luehdemann, both of 3,042,483 7/1962 Wolfram exalt ..23 2 RLudwigshafen; Wilhelm Rittinger, Schifferstadt; Artur Sliwka, Krichheim;Walter primary c Thomas saum Ludwlgshafenan of Germany AttorneyJohnston,Root, OKeeffe, Keil, Thompson & Shur- 731 Assignee: Badische Anilin- &Soda-Fabrik Aktiengesellschaft, Ludwigshafen/Rhine, Germany [57]ABSTRACT [22] Filed; Dec. 16 1968 A process for the removal of CO and/orH 8 from gases containing olefins and acetylenes by absorption in anaqueous [21 1 PP N04 783,907 solution of an alkali metal salt of aminoacid and desorption by decompressing and heating the solvent, whichcomprises [30] Foreign Application priority Dam treating the solvent,before or after desorption, with a hydrocarbon mixture substantiallyconsisting of benzene for Dec. 15, Germany ..P 7 the purpose of removingimpurities and after aid treatment Sept. 14, Germany e arating the hydroarbon mixture from the aqueous 5 1 tion. [52] US. Cl. ..23/2 R [51] Int.Cl. ..B0ld 53/34, BOld 53/16 6 Claims, 1 Drawing Figure REMOVAL OFCARBON DIOXIDE AND/R HYDROGEN SULFIDE FROM GASES CONTAINING OLEFINES ANDACETYLENES The removal of carbon dioxide and hydrogen sulfide fromsynthesis gas mixtures consisting substantially of carbon dioxide,hydrogen and carbon monoxide, by scrubbing them with aqueous solutionsof salts of amino acids is a process that has been practiced in thechemical industries for a long time. Natural gas, converter gas, watergas or gases obtained by the gasification of oils or by the pressuredevolatilization of bituminous coal, are examples of gases that can bepurified by this process.

Another known process is the removal of CO and H 8 from cracked gasesobtained when hydrocarbons are cracked to olefins, by scrubbing thesegases with a dilute solution of caustic soda. However, difficultiesarise because highly viscous polymers collect in the scrubbing liquorfrom which they must be removed for instance by filtration, separationor by a treatment with water-immiscible organic solvents to permit thecaustic soda solution to be more efl'iciently utilized by repeatedrecycling. However, this process has the serious drawback that the spentsolution of caustic soda must be subjected to a very expensive effluentprocessing treatment.

The invention concerns a development of the process for the removal ofCO and/or H 8 from gases containing olefins and acetylenes, whichcomprises contacting these gases in an absorption zone at a temperaturebetween 20 and 60C, preferably between 35 and 50C, at normal or elevatedpressure, with an aqueous solution of alkali metal salts of amino acidsadjusted to a density at 20 of 1.10 to 1.25, removing the CO and/or H Sfrom the solution leaving said zone, if desired after previouslydecompressing the same, by heating the solution in a desorption zone attemperatures between 95 ans 110C, and then recycling the solution intothe absorption zone.

The invention relates to a process of the above specified kind whichcomprises contacting the aqueous solution of alkali metal salts of aminoacids, before it enters and/or after it leaves the desorption zone, witha hydrocarbon mixture substantially consisting of benzene and itshomologues and in separating the hydrocarbon mixture from the aqueoussolution after this treatment.

The hydrocarbon treatment of the aqueous solution is generally performedat a temperature between 20 and 70C, preferably between 40 and 70C.

The new process can be used with particular advantage to thepurification of gases that are obtained in the thermal cracking ofgasolines, Diesel oils or crude oils at 700 to 950C, in the presence ofabsence of steam. Besides relatively small volumes of hydrogen sulfideand CO such gases mainly contain olefins, such as ethylene, propyleneand butene, hydrogen and methane, acetylene compounds, such asacetylene, methyl acetylene and vinyl acetylene, butadiene and otherdiolefins.

The scrubbing liquor for separating the H 8 and the CO is an aqueoussolution of alkali metal salts of amino acids, particularly of thepotassium salts of taurine, N-methyl taurine, glycocoll, a-alanine, n(/3-ethoxy) taurine, sarcosine or N-(flaminoethyl) taurine. The solutionof the potassium salt of N- methyl-a-aminopropionic acid has proved tobe particularly effective. The salts may be used individually or inadmixture. The aqueous solution is adjusted at 20C to a density of 1.10to 1.25, particularly 1.15 to 1.20. The quantity of solution to be useddepends for instance upon the amount of H 8 and CO to be removed, theconcentration of these substances in the gas mixture, the desired finalpurity, the working pressure, the working temperature and the residencetime of the gas mixture. The optimum conditions can easily be determinedfor each particular case.

It was also found that polymerization inhibitors, such as diethylhydroxylamine, trinonylphenyl phosphite or pyrogallol and particularlyalkali metal salts of nitrous acid can be added to the aqueous solutionof the alkali salts of amino acids in quantities of from to 5000 ppm,preferably 50 to 500 ppm.

The addition of between 10 and 5000 ppm, preferably between 50 and 2000ppm, of a mixture that is obtained as a residue in the distillation ofoctanols, nonanols and/or decanols produced by the 0x0 process, hasproved to be advantageous.

This mixture of substances primarily contains ethers, carboxylic estersand ketones that derive from the alcohols. It also contains minorquantities of the alcohols and sodium or potassium salts of thecarboxylic acids. Generally this mixture has an acid number between 5and 40, an ester number between 5 and 40, a ketone number between 1 and8 and a hydroxyl number between 50 and 230. The flash point is betweenabout 1 10 and 125C. The viscosity is between 2.5" and 35 Engler. Atnormal pressure about percent by volume distil at a temperature of about340. This mixture of substances may be added alone or together with theabovementioned polymerization inhibitors.

The result of this step is that the aqueous solution of the alkali metalsalts of amino acids is prevented from foaming and that thepolymerization of the readily polymerizable compounds contained in thegases is inhibited.

The process is carried out at normal or elevated pressure, preferablywithin the region of 5 to 50 ats, particularly between 15 and 35 ats.The process may be carried out in one stage or in several stages.

The hydrocarbon mixture for treating the aqueous solution should have adensity between 0.825 and 0.885 at 15C. It consists substantially ofbenzene, toluene, the xylenes, ethyl benzene and the methyl ethylbenzenes. It is preferred to use a hydrocarbon mixture that contains 60to percent by weight of aromatics having six to nine carbon atoms in themolecule. The presence of minor quantities, for instance up to a totalof about 5 percent by weight, of polymerizable compounds, such asstyrene, cyclopentadiene or methyl styrene is not objectionable in theproposed process. Moreover, the mixture may contain non-aromatichydrocarbons having from seven to about 12 carbon atoms in the molecule,in quantities up to 35 percent by weight.

The treatment of the aqueous solution of the alkali metal salts of aminoacids with the hydrocarbon mixture may be carried out immediately afterthe CO and/or H 5 has been absorbed (charged solution) or the CO and/orl-l S has been stripped (regenerated solution). Preferably the aqueoussolution of alkali metal salts of amino acids that is to be purifiedshould be thoroughly mixed with the hydrocarbon mixture. The hydrocarbontreatment can be carried out at normal or elevated pressure, forinstance up to 30 ats. The amount of hydrocarbon mixture that should beused depends for instance upon the treatment temperature, the-quantityof polymers to be removed and the thoroughness with which thehydrocarbon mixture has been mixed with the solution that is to bepurified. Generally 50 to 150 liters of the hydrocarbon mixture is usedper liters of the solution to be purified.

In order that the invention may be more readily understood, the drawingrepresents a flow sheet illustrating one embodiment of the process.

Through a pipe 1 the gas stream to be purified enters an absorptioncolumn 2. The purified gas stream leaves through a pipe 3. Theregenerated solution of salts of amino acids flows through the column 2in countercurrent and enters through a pipe 4. A pipe 5 carries thecharged liquor from the column 2 through a mixer 6 in which it is mixedwith the hydrocarbon mixture, which is supplied through a pipe 7. In aseparating tank 8 the aqueous phase is separated from the hydrocarbonphase. The hydrocarbon mixture leaves through a pipe 9. A pipe 10carries the solution charged with lbs and CO through a heat exchanger 11into a desorption column 12. In this column steam introduced through apipe 13 into the lower part of the desorption column 12 expels theabsorbed H 8 and CO as well as the hydrocarbons still dissolved in thesolution. The expelled gases are withdrawn through a pipe 14. Theregenerated solution is taken through a pipe 15 via a pump 16 to theheat exchanger 11 in which the solution transfers a major part of itsheat to the stream of liquor flowing into the desorption column. Afterhaving passed through a cooler 17 the cooled solution returns throughthe pipe 4 into the absorption column 2. lnstead of using the method ofdirect heating with steam, the desorption column 12 may be indirectlyheated with steam. in this case a valve 18 is closed, and valves 19, 20and 21 are opened. Through a pipe 22 the solution enters a boiler 23 andthen returns into the column 12 through a pipe 24. In this process steamcondensate is obtained in a condenser 25, some of which can be returnedthrough a pipe 26 and a pump 27 to the head of the absorption column 2.By adding water at this point the gas stream from which the CO and H 8has been removed can be freed from any droplets of solution it may haveentrained. When direct steam heating of the column 12 is used, water isintroduced through a pipe 28.

While the process according to this invention is carried out, no C -Chydrocarbons should be allowed to condense from the crude gas in theabsorption column 2. This can be readily prevented by ensuring that thegas from which the CO, and/or H 8 is to be removed and which enters thecolumn 2 through the pipe 1 is colder than the regenerated solution ofthe salts of amino acids entering the column 2 through the pipe 4. Ithas been found that if some of these hydrocarbons are allowed tocondense, the elimination of the polymers from the solution of the saltsof amino acids by the hydrocarbon mixture according to the invention isless good. Moreover, the formation of foam by the solution is also verymuch more pronounced.

The invention will be further illustrated by the following Examples.

EXAMPLE 1 1500 m (S.T.P.) per hour of a cracked gas substantiallyconsisting of hydrogen, methane, ethylene, propylene, 200 ppm of H S, 50ppm of CO 2.2 percent by weight of butadiene and 0.4 percent by weightof propyne and propadiene are continuously introduced at a pressure of28 ats. and a temperature of 45C into the lower part of a plate column.At the top of the column 60 liters per hour of an aqueous solution at50C of the potassium salt of N-methyl-a-aminopropionic acid, having adensity of 1.18, are continuously introduced at the same pressure. Thegas mixture emerging overhead is almost completely free from CO and H 8.The total residual content of CO and H 8 is a mere 4 ppm. The chargedsolution leaves the bottom of the column through a decompression valveand flows through a mixer, in which the solution is mixed with 5 litersof a hydrocarbon mixture having a density of 0.854 at C and a 70 percentcontent of benzene, toluene and xylenes, into an intermediate tank whichis maintained at a pressure of 2 ats. After remaining in this tank forabout 30 minutes the aqueous phase (charged solution) is decompressed tonormal pressure and taken through a heat heat exchanger to the head ofdesorption column wherein it is freed from the CO and H 8 by heating toabout 100C. The polymer content of the aqueous phase is reduced from 2.1

g/liter to 0.45 g/liter. After having been cooled and recompressed thesolution from the bottom of the desorption column is returned to thehead of the first column.

If a mixture of aliphatic hydrocarbons having five to nine carbon atomsis used instead of the proposed hydrocarbon mixture, the polymer contentof the aqueous phase can be reduced under otherwise identical conditionsto only 1.7

g/liter.

EXAMPLE 2 1500 m (S.T.P.) per hour of a cracked gas substantiallyconsisting of hydrogen, methane, ethylene, propylene, 200 ppm of H 8, 50ppm of C0 22 percent by weight of butadiene and 0.4 percent by weight ofpropyne and propadiene are continuously introduced at a pressure of 28ats and a temperature of 45C into the bottom end of a plate column. 60liters/h of an aqueous solution at 50C of the potassium salt ofN-methyl-a-amino ropionic acid having a densityl of L18 are continuouslyintro uced at the same pressure at t e top of the column. A gas mixtureemerges overhead from which the CO, and the l-l,S has been almostcompletely removed. The total residual content of CO, and H 8 is only 3ppm. The charged solution is taken through a decompression valve and aheat exchanger to the head of the desorption column. The CO, and the H 8are expelled at a temperature of 103C. The regenerated solution iswithdrawn from the bottom of the desorption column and after havingpassed through the heat exchanger this solution is thoroughly mixed with5 liters of hydrocarbon mixture at 55C and at a pressure of 3 ats. Thehydrocarbon mixture contains 40 percent by weight of benzene, 30 percentby weight of toluene, 20 percent by weight of c -aromatics and 10percent by weight of C -aromatics and has a density of 0.870 at 15C.After a separation time of 40 minutes in an intermediate tank, theaqueous phase is returned by a piston pump to the head of the absorptioncolumn. The aqueous solution has a polymer content of only 0.02 g/liter.

We claim:

1. A continuous process for the removal of CO, and/or H 5 from a crackedgas containing olefins and acetylenes as obtained in the thermalcracking of gasolines, Diesel oils or crude oils at 700C. to 950C, whichprocess comprises:

contacting said cracked gas between 20 and 60C. and

from atmospheric to superatmospheric pressure in an absorption zone withan aqueous scrubbing solution of at least one alkali metal salt of anamino acid, said solution having been adjusted to a density at 20C. ofl. 10 to 1.25 and said cracked gas having a temperature lower than thatof said scrubbing solution; removing the absorbed CO, and/or H S fromthe solution by heating the same in a desorption zone to a temperaturebetween and C. and then continuously recycling the solution to saidabsorption zone;

contacting the aqueous scrubbing solution before entering and/or afterleaving the desorption zone with a hydrocarbon mixture consistingpredominately of benzene and its homologues for the purpose of removingpolymer impurities including those formed from said olefins andacetylenes in said absorption zone and/or said desorption zone andcontained in the aqueous solution; and separating the hydrocarbonmixture containing said polymer impurities from the recycled aqueoussolution.

2. A process as claimed in claim 1 wherein the aqueous solution iscontacted with said hydrocarbon mixture at a temperature of from 20 to70C.

3. A process as claimed in claim 1 wherein a polymerization inhibitor ina quantity of from 5 to 5000 ppm is added to the aqueous solution ofalkali salts of amino acids.

4. A process as claimed in claim 1 wherein there is added to saidaqueous solution a mixture of substances which is obtained as theresidue in the distillation of octanols nonanols and/or decanolsproduced by the oxo process.

5. A process a claimed in claim 1 wherein said absorption zone ismaintained at a pressure of about 5 to 50 atmospheres and the treatmentwith the hydrocarbon mixture is carried out at atmospheric pressure upto about 30 atmospheres.

6. A process as claimed in claim 5 wherein said absorption zone ismaintained at a temperature of about 35 to 50 C. and the treatment withthe hydrocarbon mixture is carried out at a temperature of about 40 to70C.

i i 1 I 1 90-1050 UNITED STATES PATENT 0FFIE I 9 569 CETIFICATE OFGQREQTEN Date- 1 Mav 2, 1972 Patent No. 5, 660,016

In'ventofls) Harald John et a1 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Fir'st page, left-hand column, "Krichheim" should read. 1

-- Kirchheim I 6 First pa e, right-hand column, v

.3-,042,4 5 7/1962 Wolfram et al j.......23/2R" should read ,042,4837/1962 Wolfram et al ..23/2R 1,990,217 2/1935 Baehr et a1. .....-.....2/2

v FOREIGN PATENTS OR APPLICA'IlONS 1,021,546 2/1966 GreatBritain.......23/2

Column 1, line 53, "ans" should read and Column 5, 11ne52,"heat-heat"-should read heat Signed and'sealed'this 26th", day o fDecember' 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK Commissioner of Patents BEWARE M.,FLETCHER,JR.Attesting Officer

2. A process as claimed in claim 1 wherein the aqueous solution iscontacted with said hydrocarbon mixture at a temperature of from 20* to70* C.
 3. A process as claimed in claim 1 wherein a polymerizationinhibitor in a quantity of from 5 to 5000 ppm is added to the aqueoussolution of alkali salts of amino acids.
 4. A process as claimed inclaim 1 wherein there is added to said aqueous solution a mixture ofsubstances which is obtained as the residue in the distillation ofoctanols nonanols and/or decanols produced by the oxo process.
 5. Aprocess a claimed in claim 1 wherein said absorption zone is maintainedat a pressure of about 5 to 50 atmospheres and the treatment with thehydrocarbon mixture is carried out at atmospheric pressure up to about30 atmospheres.
 6. A process as claimed in claim 5 wherein saidabsorption zone is maintained at a temperature of about 35* to 50* C.and the treatment with the hydrocarbon mixture is carried out at atemperature of about 40* to 70* C.